[session] rename txstore variable to tx This is a more accurate name

[session] rename `_threaddata` to `_tx` The returned object is a Transaction object.

[session] rename self.__threaddata.txdata to self.__threaddata.tx This is Transaction object now.

[session] rename _tx_data into _txs The object inside are now Transaction object. Not TransactionData object.

[transaction] relocate method building transaction context manager It is in the middle of methods going to be refactored.

[transaction] move RQLRewriter in Transaction This remove the last ``AttributeError`` magic. The ``RQLRewriter`` needs a Session. Its reset is handled in ``Session`` code to handle that

[transaction] initialize security control attribute in Transaction There is no reason to not initialize them in __init__ time. This simplify the Session code.

[session] move security constant out of the class There is no reason for it to not be global. This will ease it use by ``Transaction object``.

[transaction] initialize hook control attribute in Transaction There is no reason to not initialize them in __init__ time. this simplify the transaction code.

[session] Move hook control constants out of the class There is no reason for them to not be global. This will ease they use by Transaction object.

[transaction] initialize transaction data and state related attribute. There is not reason to not explicitly declare and initialize attribute in Transaction.__init__. This change requires the introduction of a Transaction.clear() method to get ride of some wicked ``AttributeError`` based logic.

[transaction] handle ``mode`` default value in Transaction The transaction mode is now explicitly passed at creation time and always read from the Transaction object. Note that there is a slight behavior change. The transaction mode is now set at the creation of the transaction. Changes made to the default value have no longer any effect on existing transaction.

[transaction] explicit Transaction cnxset attribute The Transaction object have a lot more attribute that those previously declared in __init__. They are just dynamically assigned in Session code. This changeset explicit the ``cnxset`` and ``mode`` attribute declaration.

[server] rename TransactionData to Transaction Transaction will become a full featured object.

[doc] update Session documentation A major refactoring of Session/Transaction handling is commit. This is a good occasion to improves the documention.

merge 3.16 fix in default

[devtools] fix a couple issues with xvfb-run xvfb-run didn't quite clean up correctly after itself, because: - dash doesn't run EXIT traps on signals - if we don't run the command in the background, a TERM handler doesn't run until the command exits

[devtools/qunit] don't open-code subprocess.Popen.terminate

[devtools/qunit] get rid of unused variable

[server] *init_repository* lookup the database instead of the schema to drop tables (closes #810743) So, tables are dropped even if the schema was changed. :note: if mssql, drop views before tables. :note: because of table dependencies we try a second drop sequence

[book] remove XXX to an old discussion about appengine

[web publish] in case of error, ensure proper http status is set and Content-disposition header is reseted. Closes #2553066

Add CubicWebRequestBase.content (closes #2742453) This is the body of the HTTP request (stream)

add the match_http_method predicate (closes #2559932) This aim at creating views which are selected on other HTTP methods (PUT, DELETE, ...)